Tendons transmit the force generated by muscle contraction to the skeleton, through robust and highly organized bundles of collagen fibrils that establish the biomechanical properties of tendons. In the tendons, cellular extensions engulf bundles of collagen fibrils and generate the microenvironment for fibril growth. Growth of the collagen fibrils in tendons is the single most significant factor that determines biomechanical properties and tensile strength of tendons. Collagen fibril growth occurs in two phases, slow fibril assembly in embryonic stages and a much faster pace of fibril growth that likely occurs through fibril fusion in postnatal stages. Nothing, however, is known about the genetic program that regulates these processes. We have previously shown that the bHLH transcription factor Scleraxis is essential for early tendon differentiation and that it likely also plays an important role in collagen fibrillogenesis. In mutants of a second tendon transcription factor, Mohawk, we now find a failure of the later postnatal phase of collagen fibril growth. This project focuses on the role of Scleraxis and Mohawk in collagen fibrillogenesis and tendon maturation. The first specific aim focuses on tendon assembly in embryonic tendons and the second aim looks at the regulation of tendon maturation and rapid collagen fibril growth in postnatal stages. These processes will be addressed following a similar approach in both stages. Normal tendon growth and tendon phenotypes will initially be evaluated with an enhanced set of structural parameters and the effects of overexpression of Scx or Mkx on tendon growth will be examined. Molecular mediators of Scleraxis and Mohawk functions will be identified by microarray profiling and the regulatory roles of a small number of target genes will be determined by a transgenic rescue of the Scx or Mkx phenotypes reintroducing the expression of a single or multiple target genes into the mutant background. We recently identified ZFP185, a Zinc Finger Transcription factor as the first promising candidate for which we plan to proceed with a transgenic rescue. Identifying regulatory pathways that control fibril growth will likely contribute to the ability to enhance and regulate these processes in clinical settings.